Gasoline engine with an exhaust system for combusting particulate matter

ABSTRACT

A gasoline engine having an exhaust system comprises means for trapping particulate matter (PM) from the exhaust gas and a catalyst for catalysing the oxidation of the PM by carbon dioxide and/or water in the exhaust gas, which catalyst comprising a supported alkali metal. The invention further includes a method of combusting PM from a gasoline engine in CO 2  and/or H 2 O from the exhaust gas at temperatures in excess of 500 ° C., which method comprising trapping the PM and contacting it with a catalyst comprising a supported alkali metal.

The present invention relates to an exhaust system for treating exhaustgases from a gasoline engine, and in particular it relates to an exhaustsystem for trapping and combusting fine particulate matter in theexhaust gas.

It is very well known that gas-borne particulates can cause healthproblems if inhaled. It is also well known that diesel engines emitparticulates, especially upon start-up and under heavy load, and thiscan be observed with the naked eye. Concern about particulates has ledto the introduction of various National or European regulations tocontrol the quantity of particulates emitted from diesel engines.Johnson Matthey has patented and commercially introduced a device calleda CRT™ for diesel engines, particularly heavy-duty diesel engines ((asdefined by the relevant European, US Federal or Californian legislation)see EP-B-341832 and U.S. Pat. No. 4,902,487) which has made available aself-regenerating diesel filter device.

However, it is not generally appreciated that many modem gasolineengines emit large numbers of very small (<0.1 μm generally 10-100 nm)particulate matter (PM), which because of its small size is not observedby the naked eye. Nonetheless, the size of this PM is such that it canbe inhaled into the deepest recesses of the lungs and may even be morehazardous than the PM from diesel engines. Indeed, as reported in theProceedings from the 2^(nd) International Conference on Health Effectsof Vehicle Emissions, Ed. K. Donaldson, (2000), PM poses a significantthreat to health in the UK. Over 8000 deaths are thought to be relatedto this form of emission. It has been suggested that particles in thesize range 10-100 nm, of which 60% of those in the atmosphere arevehicle derived, penetrate deep inside the lungs causing respiratory andcardiovascular disease.

We believe that the latest types of lean-burn gasoline engines, such asdirect injection e.g. GDI and direct injection spark-ignition (DISI),produce large amounts of fine PM. Furthermore, we believe that the fuelefficiency of such lean-burn gasoline engines will result in theirincreasing use in private cars and small utility vehicles. Also, recentresearch indicates that there is considerable PM generation from portfuel injection or stoichiometrically operated (air/fuel ratios ofapproximately 14.7:1) gasoline engines at high load, such as during highspeed driving or ascending a steep slope. In particular, at such highloads it is found that the number of PM in the exhaust is equivalent tonormally operated diesel engine, although the mass of PM is much lessthan in diesel exhaust. (See “Study of the number, size and mass ofexhaust particles emitted from European Diesel and Gasoline vehiclesunder steady-state and European driving cycle conditions”, Concawe,report no. 98/51).

Typically, average exhaust gas temperatures for a stoichiometricallyoperated gasoline engine are between 600-800° C. and for GDI betweenabout 300-550° C., although at high speed, GDI engines can revert tostoichiometric operation and hotter exhaust gas temperatures arereached. Exhaust gas temperatures from diesel engines, however, are muchcooler than gasoline engines. Typically, for passenger vehicles usinglight-duty diesel engines, exhaust gas temperatures are in the rangefrom 200-350° C. and, for a heavy-duty diesel plant, about 200-550° C.

In seeking to combust gasoline PM, one problem is that the amount ofoxidant available in the exhaust gas for combusting the PM downstream ofa three-way catalyst (TWC) is generally very low at approximately 0.03%v/v (300 ppm). By comparison diesel exhaust gas typically includesbetween 2-12% O₂ v/v at maximum output (up to 18% O₂ v/v at idle). Modemgasoline vehicles, including stoichiometric and lean-burn engines,typically include a TWC in the close-coupled position, i.e. close to theexhaust manifold. This is to treat exhaust gas in the period immediatelyfollowing start-up, whereby the close-coupled catalyst benefits fromheat generated by the combustion event to more rapidly reach light-offtemperature than a catalyst in the underfloor location.

Two possible sources of oxygen in gasoline exhaust gas include watervapour (present at about 9% v/v) and carbon dioxide (about 18% v/v). Themethod of using water vapour as an oxidant is known as steamgasification and the reverse-Boudouard reaction is the term given to theoxidation of species with CO₂. General equations for the steamgasification of carbonaceous PM are represented by equations (1), (2)and (3) and for the reverse-Boudouard reaction by equation (4).C(s)+2H₂O(g)→CO₂(g)+2H₂(g)  (1)2C(s)+2H₂O(g)→CO₂(g)+CH4(g)  (2)C(s)+H₂O(g)→CO(g)+H₂(g)  (3)C(s)+CO₂(g)→2CO(g)  (4)

A common device using electrostatic fields to remove suspended particlesfrom a gas is known as the electrostatic precipitator (ESP). Particlesentering an ESP are charged in an electric field and then moved to asurface of opposite polarity where deposition occurs. Electrostaticprecipitators consist mainly of a grounded cylinder (the collectingelectrode) and a coaxial high potential wire (the corona dischargeelectrode). An alternative basic design consists of two groundedparallel plates (the collecting electrode) together with an array ofparallel discharge wires mounted in a plane midway between the plates.If a sufficient potential difference exists between the discharge andthe collecting electrodes, a corona discharge will form. The coronaserves as a copious source of unipolar ions of the same sign as that ofthe discharge electrode; the aerosol particles are then charged by ioncollision and attracted to the collecting surface.

GB-A-2232613 describes a catalyst for the oxidation of carbonaceousparticulates carried in a gaseous phase and exemplifies its use intreating diesel engine exhaust emissions. The catalyst comprises ceriaand a ceramic oxide (preferably alumina) with a compound of an elementof Group 1A of the periodic table. The catalyst is made by combining asol of the ceramic oxide with another of the cerium (IV) oxide and thenadding a dispersion of a compound of Group 1A. The mixture is coated toa substrate, dried and calcined. An Example is described wherein theGroup 1A compound is K₂O or Rb₂CO₃. The onset temperature (defined asthe temperature where carbon just begins to oxidise significantly andburn to CO₂) for PM collected from a diesel engine is 364° C. and 335°C. respectively. The document does not mention gasoline engines as asource for PM or of the use of the catalysts described for treatinggasoline-derived PM.

U.S. Pat. No. 5,837,212 describes a lean NOx trap comprising a poroussupport; and catalysts consisting of manganese and potassium loaded onthe support. During lean-burn operation of the engine the trap sorbs NOxand releases NOx during decreased oxygen concentration in the exhaustgas. There is no suggestion in the document that the catalysts can beused in conjunction with a gasoline particulate trap.

U.S. Pat. No. 6,038,854 describes an internal combustion exhaustconnected by a pipe to a chamber where carbon-containing particles areelectrostatically trapped or filtered and a non-thermal plasma convertsNO to NO₂ in the presence of O₂ and hydrocarbons. Volatile hydrocarbons(C_(x)H_(y)) from the trapped particulates react with the NO₂ to convertNO₂ to N₂, and the soot to CO₂. Illustrated embodiments include aplasma-assisted particulate trap; a plasma reactor upstream of aparticulate trap; a plasma reactor upstream of a particulate trapreactor which is upstream of a catalytic reactor; and a particulate trapcylindrically concentric about a plasma reactor therewithin and,optionally, having a catalytic converter cylindrically concentric aboutthe trap.

We have now found, very surprisingly, that catalysts comprising at leastone alkali metal are active for oxidising gasoline PM with CO₂ and H₂Oat temperatures within the typical temperature range of exhaust gas froma gasoline engine. By “gasoline engine” herein, we include bothstoichiometric and lean-burn engines, such as direct injection gasolineengines, e.g. GDI and DISI engines.

According to one aspect, the invention provides a gasoline engine havingan exhaust system, which exhaust system including means for trappingparticulate matter (PM) from the exhaust gas and a catalyst forcatalysing the oxidation of the PM by carbon dioxide and/or water in theexhaust gas, which catalyst comprising at least one supported alkalimetal.

In use, we believe that the at least one alkali metal can be present aselemental metal, its oxide, its hydroxide or its carbonate, althoughpossibly also as its nitrate or its sulfate. Whilst the catalysts of theinvention are especially suited to catalysing the oxidation of PM withCO₂ and/or H₂O, it will be appreciated that such catalysts are alsocapable of catalysing the oxidation of PM with any O₂ present in theexhaust gas, for example as seen in GB-A-2232613. The combustion of PMin O₂ can occur at lower temperatures than in CO₂ and H₂O.

Preferably, the at least one alkali metal is lithium, sodium, potassium,rubidium or caesium or a mixture of any two or more thereof, preferablypotassium. Where the at least one alkali metal is a mixture, we preferthat the mixture is a eutectic mixture.

The elemental alkali metal can be present in the catalyst at from 1% to20% by weight of the total catalyst, preferably from 5-15% by weight andmost preferably 10% by weight.

The support for the at least one alkali metal can be alumina, ceria,zirconia, titania, a silica-alumina, a zeolite or a mixture or a mixedoxide of any two or more thereof. Advantageously, we prefer that thesurface area of the support is as low as possible. This is because webelieve that the alkali metal catalyst can be mobile, e.g. molten, atits working temperatures. If the support is porous, the mobile alkalimetal can migrate into the pores away from the surface of the support,thus effectively reducing the surface area of the alkali metal and withit the activity of the catalyst. Thus, where the support is alumina, weprefer to use alpha-alumina (surface area 1-5 m² g⁻¹) or theta-alumina(90 m² g⁻¹), although gamma-alumina (140 m² g⁻¹) can also be used, or amixture of any two or more thereof

Most preferably, the at least one alkali metal is potassium and thesupport is alpha-alumina, zirconia or ceria or a mixture or mixed oxideof any two or more thereof.

Where the support is a mixed oxide, each component oxide of the mixedoxide can be present in an amount of from 10% to 90% by weight of totalcatalyst weight. For example, in a binary mixed oxide, the ratio of thecationic components can be from between 20:80 and 50:50 by weight of thetotal catalyst.

The substrate for the supported at least one alkali metal can be theinternal surface of a conduit carrying the exhaust gas which conduit canbe of metal or ceramic, or a metal or ceramic flow-through or wall-flowfilter monolith, foam, e.g. metal oxide foam, or wire mesh.

The means for trapping can comprise any apparatus suitable for trappingthe PM of the appropriate particle size, e.g. between about 10-100 nm.Such means can include a wall-flow filter made from a ceramic materialof appropriate pore size such as cordierite. Where a filter is used, thefilter can be coated, at least in part, with the catalyst in order toeffect contact between the catalyst and the PM.

Alternatively, the trapping means can comprise a foam e.g. a metal oxidefoam, wherein the foam can act as a filter. The foam per se can comprisethe support, i.e. the foam is both supports and substrate, or the foamcan be the substrate and a support e.g. comprising a high surface areametal oxide washcoat, can be coated thereon. Any suitable metal oxidefoam can be used, but we prefer materials having the sufficientrobustness for use in exhaust systems. Non-limiting examples of foamsinclude alpha-Al₂O₃, ZrO₂/MgO, ZrO₂/CaO/MgO and ZrO₂/Al₂O₃.

Another substrate is a wire mesh and this can comprise the support, i.e.the wire mesh can be both support and substrate, or it can be asubstrate to which a catalyst support is coated. Where the wire mesh isboth support and substrate, suitable pre-treatment of the wire mesh canbe used to “key” the coating to the wire mesh. Such techniques are knownin the art and include a sol base layer or a NaOH(aq) surfacepre-treatment. These techniques can also be used to “key” the catalystto metal surfaces such as the flow-through monolith and conduit surfacesdescribed above.

Alternatively, more conventional filter technology such as wall-flowfilters used for diesel applications and well known to persons skilledin the art can be used as a catalyst support. A further, less preferred,trapping means utilises thermophoresis and is described in ourGB-A-2350804.

Alternatively, or in addition, the trapping means can comprise adischarging electrode and a collecting electrode for electrostaticdeposition of the PM and power means for applying a potential differencebetween the discharging electrode and the collecting electrode, i.e. thedeposition method involves electrophoresis.

In embodiments of the present invention, geometries of the dischargingand collecting electrodes can include: (i) a first cylinder and a wiredisposed coaxially therein; (ii) in a variation of the embodiment at(i), a plurality of further wires extending longitudinally relative tothe cylinder, which wires are arranged equidistantly and radially aboutthe coaxially disposed wire; (iii) in a variation on the embodiments at(i) and (ii), the first cylinder comprises inner and outer surfaces andan insulating layer is disposed therebetween, which first cylinder iscoaxially disposed within a second cylinder, the arrangement being suchthat the power means is capable of applying a potential differencebetween the coaxial wire and the inner surface of the first cylinder andbetween the outer surface of the first cylinder and the second cylinder;(iv) a plurality of first or second cylinders of embodiment (i), (ii) or(iii) above arranged in parallel; and (v) a pair of plates arranged inparallel and a plurality of wires arranged equidistantly in parallel toeach other and disposed midway between the plates.

In one embodiment according to the invention, the collecting electrodein the electrophoretic trapping means can be earthed. In embodiment (i),(ii) and (iv) above, the collecting electrode can be the or each firstcylinder; in embodiment (iii), the collecting electrode can be the innersurface of the first cylinder and the second cylinder; and in embodiment(v), the collecting electrodes can be the plates.

In the electrophoretic trap embodiments of the present invention, thecatalyst can be coated, at least in part, on the collecting electrode.

In a particularly preferred embodiment, the first or second cylinder isa section of the exhaust pipe carrying the exhaust gas. This arrangementhas the advantage that it is relatively non-intrusive to the typicalconfiguration of a vehicle exhaust system, i.e. the presence of thecoaxial wire(s) and the means for holding the coaxial wire in itscoaxial arrangement and for holding any additional wires parallelthereto, e.g. ceramic insulators or ceramic tubes, do not alter thepressure drop in the exhaust pipe. Accordingly, there is little or noimpact on the efficiency of the engine.

An advantage of the present invention incorporating an electrostatictrap, such as the cylinder-wire embodiments, is that they have arelatively low power consumption e.g. about 10W, therefore there isminimal impact on the overall efficiency of the vehicle.

According to a further aspect, the invention provides a method ofcombusting PM from a gasoline engine in CO₂ and/or H₂O from the exhaustgas at temperatures in excess of 500° C., which method comprisingtrapping the PM and contacting the trapped PM with a catalyst comprisinga supported alkali metal. The temperatures in excess of 500° C can bee.g. 550° C., 600° C., 650° C. or 700° C.

In order that the invention may be more fully understood, the followingExamples are provided by way of illustration only wherein reference ismade to the accompanying drawings in which:

FIG. 1 is a bar chart of temperature (° C.) against oxygen sourcecomparing the activity of a 10 wt % K/Ceria catalyst with a control;

FIG. 2 is a schematic diagram of a laboratory simulation test rig fortrap evaluation;

FIG. 3 is a trace showing time-resolved particle number density from thefour lowest-size stages (channels) of an electrical low pressureimpactor (ELPI), during five minute portions of a nominal steady statetest at the equivalent road speed of 80 km/h performed on a 1.8 litreGDI engine fitted with a wall flow particulate filter;

FIG. 4 shows the wall-flow filter performance: time variation ofparticle number density, of the FIG. 3 system over the final ten minutesof the European ECE+EUDC drive cycle for two sizes (in nm) denoted bynumber beside the curves;

FIG. 5 is a bar chart showing the performance of a cylinder-wireelectrostatic precipitator to trap PM of four sizes (in nm) measured atcylinder outlet over a range of potential differences applied betweenthe grounded cylinder and the wire relative to a control; and

FIG. 6 shows the performance, as represented by % collection efficiencyby number against mobility diameter D _(p) (nm), of a wire-cylinder trapwherein the inner surface of the cylinder is coated with a catalystaccording to the invention compared to an identical uncoated device.

EXAMPLE 1 Catalyst Preparation

A 10 wt % K/Al₂O₃, 10 wt % K/CeO₂ and 10 wt % K/ZrO₂ (as the elementalalkali metal based on the total weight of the catalyst) was prepared bywet impregnation. In each case the impregnation medium was an aqueoussolution of KNO₃. A mixture of the correct amounts of the support andimpregnation solution was heated to evaporate the water and the materialwas calcined at 500° C. for 2 hours. Three alumina supports were used:alpha-, theta- and gamma-. We understand that the alkali metal ispresent as K₂O in each catalyst, although some residual KNO₃ may bepresent post-calcination.

EXAMPLE 2 Catalyst Ageing

Pelletised catalysts of Example 1 were aged in a simulatedstoichiometric gasoline exhaust gas mixture of nitrogen, water, carbonmonoxide, hydrogen, oxygen, sulphur dioxide at 850° C. for 2 hours and16 hours.

EXAMPLE 3 Preparation of Catalyst/Particulate Matter Samples

Fresh catalyst of Example 1 and aged catalyst from Example 2 were eachmixed with about 30-40% w/w carbonaceous PM (BP2000—high surface areagraphite, a simulant for gasoline exhaust PM) lightly in a mortar andpestle to ensure thorough mixing. Light mixing was employed in order tosimulate real conditions in an exhaust system in which the gasoline PMwould loosely contact a trap device including a catalyst. Thus heavygrinding, which would promote too tight a contact between the PM and thetrap, was avoided.

EXAMPLE 4 Sample Testing

The samples of Example 3 were tested in synthetic gas streams includingoxygen, water or CO₂ to simulate the temperature and concentration ofoxygen, water and CO₂ components found in a gasoline exhaust gas. Thisapproach has the advantage that oxidation in oxygen and the steamgasification and reverse-Boudouard reactions can be studied inisolation, whereas in practice the exhaust gas will comprise a mixtureof oxygen, water vapour and CO₂ among others and the three processes canoccur simultaneously.

The samples of Example 3 were tested using Thermogravimetric Analysis(TGA) to determine the temperature where significant weight loss of thesamples occurred, indicating the point where combustion or any phasechanges within each sample took place. The technique operates using amicrobalance, which measures weight change as a function of temperature.When the combustion temperature is reached, the carbon is removed aseither CO₂ or CO and therefore the weight of the sample decreases.

Temperature Programmed Oxidation (TPO) was used to measure the effect ofwater as a source of oxygen for combustion. TPO involves passing heliumthrough a water saturator at room temperature and then over thecatalyst. The production of CO₂ and CO is then monitored as a functionof temperature.

The conditions used for each of these techniques are described inTable 1. TABLE 1 Conditions used for the Combustion Reactions Flow-ratesFinal Temp. Ramp-rates Technique Gases (ml min⁻¹) (° C.) (° C. min⁻¹)TGA Air 100 700 15 10% CO₂/Ar 100 1200 15 Ar 100 1200 15 TPO H₂O/He75-80 900 10

The results for the samples are given in the attached Tables 2, 3 and 4.FIG. 1 summarises the results for 10 wt % K/Ceria, showing that thetemperatures for combustion of PM in oxygen, water and CO₂ are withinthe normal operating temperature of a gasoline exhaust system for aEuropean passenger vehicle.

Reactions involving gas phase reactants can use conventionalheterogeneous catalysts. However, catalysts for reacting solid carbonwith carbon dioxide, steam or oxygen, preferably have differentproperties from conventional heterogeneous catalysts. In order tocatalyse the reaction with solid carbon it is understood that thecatalyst should come into contact with the carbon material to create aboundary across which oxygen can be transferred. To achieve this, it ispreferred that the active component of the catalyst is mobile atreaction temperature. This is illustrated by comparison of thepotassium-based catalysts with a Pt/Zirconia catalyst. Combustion inoxygen using 10 wt % K/Zirconia is initiated at 391° C., (an improvementon the non-catalytic reaction that takes place at approximately 550°C.). In comparison, when 1 wt % Pt/Zirconia is used, temperatures ashigh as 523° C. are required. The reason behind this observation isprobably that at 350° C. the active potassium oxide begins to melt andso becomes mobile although still retained within the pore structure ofthe zirconia. In contrast, platinum oxide decomposes at approximately550° C., and the metal particles formed are immobile until highertemperatures are reached, consequently limiting the contact betweencatalyst and soot at lower temperatures.

As can be seen from the results shown in Tables 2, 3 and 4, thecatalysts of the present invention reduce the combustion temperature ofsimulated gasoline PM to gasoline exhaust gas temperatures. Furthermore,it can be seen that lower surface area supports retain catalyst activityat lower temperatures following high temperature ageing. This isbelieved to be because the metal compounds are mobile at activetemperatures and they can migrate deep into the smallest pores of highersurface area supports, thereby reducing the effective surface area ofthe alkali metal and making it less available to catalyse the relevantreaction.

Furthermore, the results in the Tables 2, 3 and 4 show that the supportmaterial can have a role to play in the chemistry of the process. Theinitial combustion temperatures of the reactions of oxygen, water andcarbon dioxide with carbon can be reduced further when using 10 wt %K/Ceria and 10 wt % K/Zirconia than for α-alumina supported catalyst.The benefit is most noticeable in the case of 10 wt % K/Ceria for thereaction of carbon dioxide with PM: combustion starts to take place at712° C. for the fresh catalyst, increasing slightly to 746° C. and 748°C. for the 2 hours and 16 hours aged samples. In the case of 10 wt %K/Zirconia, the most significant improvement occurs in the steamgasification reaction. Initial combustion is detected at 477° C. for thefresh catalyst again increasing slightly to 477° C. and 532° C. for the2 hours and 16 hours aged samples.

It should be noted that all of the above catalysed reactions, with theexception of 1 wt % Pt/Zirconia, occur at significantly lowertemperature than the non-catalysed reactions. This improvement could bedue to the redox properties of these support materials i.e. both ceriaand zirconia can undergo oxidation or reduction readily depending on theconditions (CeO₂⇄Ce₂O₃, ZrO₂⇄Zr₂O₃).

Another property of the catalysts of the present invention is that it isunderstood that they can activate carbon dioxide and water in order totransfer the oxygen to trapped PM. At present the reason for thisimprovement is not fully understood, but we believe it is the ability ofthe alkali metals to form reactive intermediates in the presence ofcarbon dioxide and water that make them the most suitable catalysts forthis purpose. Again, comparison of the potassium based catalysts with 1wt % Pt/Zirconia, relative to the non-catalysed reactions, show that thealkali metals can reduce the initial combustion temperatures by up to300° C. unlike the platinum catalyst, which only reduces the temperatureby approximately 30-50° C.

EXAMPLE 5 Particulate Filters

25 mm-diameter×152 mm length cores were extracted from Corning EX-80100/17 cordierite wall-flow filter.

Tests in Laboratory Flow Rig

Carbon particles were generated by spark discharge between graphiteelectrodes in a Palas GFG1000 carbon aerosol generator (CAG). 10-200 nmagglomerates from the CAG, in an inert carrier/dilution gas(argon/nitrogen), were then dispersed in a synthetic gas mixture whichcan include water and hydrocarbon vapours. While the elemental carbonfraction of particulates from PFI engines is normally not more than˜40%, considerable variability from similar light-duty gasoline vehicleshas been found, with significant elemental carbon emissions in somecases. GDI engines can produce particulates more closely resemblingthose from diesel engines, up to 72% carbon being reported, so thissimulation approach is particularly relevant to GDI engines.

The particle-laden gas flowed in a straight, 50 mm diameter, insulated,stainless steel pipe, 3 m in length, to which after-treatment devicescould be attached in-line. An in-line process heater and heating tapewere used to achieve the desired uniform gas temperature. Particles weresampled using 6.5 mm stainless steel probes and measurements of numberconcentration and size distribution were made using a TSI ScanningMobility Particle Sizer (SMPS) comprising a model 3081L electrostaticclassifier (EC) and a model 3022 condensation particle counter (CPC). Tokeep sample temperature below the 36° C. limit, samples were dilutedwith particle-free nitrogen in a three-stage ejector system.

The laboratory simulation test rig for trap evaluation is shownschematically in FIG. 2.

For the tests reported here, the synthetic gas was composed only ofparticle-free air together with the CAG carrier/dilution gas. Tests werecarried out at a fixed gas mass flow rate and temperatures T betweenambient and 400° C. and for filter exposure times up to 20 hours.[Temperatures were limited to a maximum of 400° C. by the heatingsources used in the laboratory test rig. However, the trends shown inthe results obtained allow us to predict system performance at highertemperatures, e.g. at least 500° C.] The chosen gas flow rate andparticle production rate provided particle mass concentrations of 1.06mg/m³ and 0.46 mg/m³ at T=20 C. and 400° C. respectively, within therange 0.1-10 mg/m³ typical of gasoline engine exhausts. The filter spacevelocity (gas actual volume flow rate divided by filter volume) wasapproximately 44000 h⁻¹ at 20° C. and 101000 h⁻¹ at 400° C. Measurementswere taken at locations 100 mm upstream and downstream of the filter.The SMPS particle size window was set to 9-422 nm and scan times for upand down scans were chosen as 120 seconds and 60 seconds, respectively;the time interval between measurements upstream and downstream was ˜5min. A sample dilution ratio of 60 was used.

The results obtained are shown in Table 5. TABLE 5 Total number particleNumber particle Air flow rate concentration concentration Temperature(L/min at average reduction average peak (° C.) T = 22° C.) (%)reduction (%) 22 35 54 50 200 35 57 54 400 35 54 46

At 400° C., the measured capture efficiency as a function of particlesize was between 65% and 90% throughout the ultrafine size range, withincreased scatter below 20 nm where measurement uncertainty was expectedto be highest and above 100 nm where the smallest numbers of particleswere detected. Little change in the pattern was seen after an exposuretime of 19 hours, when the pressure drop had increased by a factor ofmore than four. (With effective continuous or intermittent oxidation oftrapped PM, pressure drop should be of less concern.) The time-averagedsize distributions show no shift in mode size across the filter

TESTS ON GDI ENGINE

A filter of identical type, but with 118 mm diameter, was fitted to theexhaust system of a Mitsubishi Carisma car with 1.8 litre GDI engine,for comparison with the standard exhaust system. The filter location wasapproximately one third of the way between engine exhaust manifold andtailpipe exit. With the car on a chassis dynamometer, raw exhaustsamples were extracted at the tailpipe though a stainless steel samplingprobe and line. Samples were first diluted in a Dekati two-stageDiluter, with a dilution ratio of 74, then analyzed using a DekatiElectrical Low Pressure Impactor (ELPI), a 12-channel real-time particlesize spectrometer covering the size range 0.03-10 μm.

Measurements with and without the filter were made at two conditions:(a) nominally steady state at an equivalent road speed of 80 km/h, wherefuel injection was in the stratified overall-lean mode with a measuredequivalence ratio of around 1.8; (b) transient, over the EuropeanECE+EUDC drive cycle. FIG. 3 shows the time-resolved particle numberdensity from the four lowest-size stages (channels) of the ELPI, duringfive-minute portions of the steady-state tests. The baselinemeasurements (dashed curves, left-hand scale) were taken immediatelyafter the car had been taken through a drive cycle, and the plotincludes periods in both 4th and 5th gears. With the filter fitted andpre-loaded during one hour of operation (solid curves, right-handscale), the number density had fallen by an order of magnitude, for allsizes in the range 30-170 nm. Beyond 170 nm, particles were not detectedin significant numbers, with or without the filter. Thus the filterappeared to perform as well as in the laboratory simulation, anunsurprising result.

Over the ECE and lower-speed portion of the EUDC cycle, the filterreduced tailpipe number concentrations in the ultrafine range to valuesvery significantly below the baseline levels, as illustrated in FIG. 4.No significant change in filter performance over the cycle was seenbetween filters pre-loaded for 1 hour and for 4 hours.

EXAMPLE 6 Electrostatic Separators

A wire-cylinder electrostatic trap consisting of a groundedstainless-steel pipe of 50 mm diameter and 1 m length with an axialdischarge electrode of 0.1 mm tungsten wire was constructed. Thedimensions were selected on the basis of estimates of the axial distancerequired for 100% particle capture at typical gas velocities in alaboratory simulation, supported by analytical modelling. The wire wasenergized using a Start Spellman SL300 high voltage power supply andkept both taut and centrally located by an insulated tensioning devicewhich allows the trap to be installed in-line with a simulated exhaustpipe, in place of the filter shown in FIG. 2. Particle charging wasexpected to occur through ion bombardment in the corona discharge, priorto transport to the pipe wall by electrophoretic drift.

A simulated gasoline engine exhaust (generated as described in Example5) dispersed in particle-free nitrogen, was fed to the trap at ambienttemperature. The residence time within the trap was <1s, ensuringturbulent flow. With the field, at V=7 kV and with a corona currentaround 1 mA, the particle numbers at outlet were many orders ofmagnitude smaller than at inlet, over most of the size range and for allthe radial sampling positions (measured within a 15 min period). In the10-20 nm range, the reduction in particle number was still two orders ofmagnitude. Evidence that particles had been deposited, rather thanremaining gas-borne with a radial shift, was provided by observation ofa thin, uniform, black deposit on the trap wall after some 3 hours ofrunning.

The experiment was repeated over a range of voltages and the results areshown in FIG. 5. As can be seen, the particulate separation in thisarrangement reaches a plateau above about 12 kV.

EXAMPLE 7 Catalysed ESP

A simulated gasoline engine exhaust was heated to 400° C. and fed to thetrap (described in Example 6) at that temperature. The residence timewithin the trap was 0.6 s, ensuring a realistic turbulent flow. With theelectric field maintained at 8 kV, the particle trapping efficiency wasmeasured as a function of particle diameter. After completion of thistest, the simulated exhaust pipe (see FIG. 2) was replaced by anidentical pipe which had been coated in 10 wt % K/gamma-aluminacatalyst. The results, shown in FIG. 6, appear to indicate that thecatalyst does not materially affect the performance of the separator,and if anything, provides a slight improvement in collection efficiencyfor larger particle sizes. TABLE 2 Combustion Characteristics for Freshand Aged Catalysts in O₂ (using TGA) 2-Hour Aged 16-Hour Aged FreshInitial Initial Initial Combustion Combustion Combustion 2-Hour Aged T₅₀16-Hour Aged Catalyst Temp. (° C.) Temp. (° C.) Temp. (° C.) Fresh T₅₀(° C.) (° C.) T₅₀ (° C.) None 550 — — 625 — — 10% K/γ-Alumina 410 510527 448 560 579 10% K/θ-Alumina 414 502 499 444 551 548 10% K/α-Alumina410 461 468 439 564 542 10% K/Ceria 407 499 479 444 574 576 10%K/Zirconia 391 479 505 432 564 593Key: T₅₀ is the temperature at which 50% of the weight of soot isremoved.

TABLE 3 Combustion Results for Fresh and Aged catalyst in CO₂ (usingTGA). 2 hour-aged 16 hour-aged Fresh Initial Initial Initial CombustionCombustion Combustion 2 hour-aged T₅₀ 16-hour aged T₅₀ Catalyst Temp. (°C.) Temp (° C.) Temp (° C.) Fresh T₅₀ (° C.) (° C.) (° C.) None 979 — —1130 — — 10% K/γ-Al₂O₃ 743 808 836 808 891 917 10% K/θ-Al₂O₃ 743 773 813804 888 923 10% K/α-Al₂O₃ 731 739 757 808 819 819 10% K/CeO₂ 712 746 748786 817 832 10% K/ZrO₂ 730 748 763 804 815 836Key: T₅₀ is the temperature at which 50% of the weight of soot isremoved.

TABLE 4 Combustion Results for Fresh and Aged catalyst in H₂O (usingTPO). 2 hour-aged 16 hour-aged Fresh Initial Initial Initial CombustionCombustion Combustion 2 hour-aged T₅₀ 16-hour aged T₅₀ Catalyst Temp. (°C.) Temp (° C.) Temp (° C.) Fresh T₅₀ (° C.) (° C.) (° C.) None 850-900— — >900 — — 10% K/γ-Al₂O₃ 505 694 673 751 >900 >900 10% K/θ-Al₂O₃ 525699 690 709 >900 >900 10% K/α-Al₂O₃ 505 585 669 736 840 >900 10% K/CeO₂550 597 599 753 773 815 10% K/ZrO₂ 477 477 532 755 759 823Key: T₅₀ is the temperature of half-peak maximum temperature.

1. A gasoline engine having an exhaust system, which exhaust systemcomprising a three-way catalyst (TWC), means for trapping particulatematter (PM) of <100 nm from the exhaust gas and a catalyst forcatalysing the oxidation of the PM by carbon dioxide (CO₂) and/or water(H₂O) in the exhaust gas, which catalyst comprising a supported alkalimetal, wherein the supported alkali metal is disposed downstream of theTWC.
 2. An engine according to claim 1, wherein the alkali metal isselected from the group consisting of lithium, sodium, potassium,rubidium, caesium and mixtures of any two or more thereof.
 3. An engineaccording to claim 2, wherein the mixture is a eutectic mixture.
 4. Anengine according to claim 1, wherein the elemental alkali metal ispresent in the catalyst at from 1% to 20% by weight of the totalcatalyst.
 5. An engine according to claim 1, wherein the support isselected from the group consisting of alumina, ceria, zirconia, titania,a silica-alumina, a zeolite, a mixture thereof and a mixed oxide of anytwo or more thereof.
 6. An engine according to claims 1, wherein thesupport is alumina selected from the group consisting of alpha-alumina,theta-alumina, gamma-alumina and mixtures of any two or more thereof. 7.An engine according to claim 1, wherein the alkali metal is potassiumand the support is selected from the group consisting of alpha-alumina,zirconia, ceria and mixtures of any two or more thereof.
 8. An engineaccording to claim 5, wherein the support comprises a mixed oxide andeach component of the mixed oxide is present in an amount of from 10 wt% to 90 wt % by total catalyst weight.
 9. An engine according to claim8, wherein the mixed oxide is a binary mixed oxide and the ratio of thecationic components present is in a ratio of from 20:80 to 50:50.
 10. Anengine according to claim 1, wherein the trapping means comprises afilter.
 11. An engine according to claim 10, wherein the filtercomprises a wall-flow filter, a foam or a wire mesh.
 12. An engineaccording to claim 11, wherein the wall-flow filter is the foam and thefoam is a metal oxide.
 13. An engine according to claim 1, wherein thetrapping means comprises a discharging electrode and a collectingelectrode for electrostatic deposition of the PM and power means forapplying a potential difference between the discharging electrode andthe collecting electrode.
 14. An engine according to claim 13, whereinthe trapping means comprises a first cylinder and a wire disposedcoaxially therein.
 15. An engine according to claim 14, wherein thetrapping means further comprises a plurality of wires extendinglongitudinally relative to the first cylinder, which wires are arrangedequidistantly and radially about the coaxially disposed wire.
 16. Anengine according to claim 14, wherein the first cylinder comprises innerand outer surfaces and an insulating layer is disposed therebetween,which first cylinder is coaxially disposed within a second cylinder, thearrangement being such that the power means is capable of applying apotential difference between the coaxially disposed wire and the innersurface of the first cylinder and between the outer surface of the firstcylinder and the second cylinder.
 17. An engine according to claim 14,comprising a plurality of first or second cylinders arranged inparallel.
 18. An engine according to claim 14 further comprising meansfor holding the coaxial wire in its coaxial arrangement and anyadditional wire parallel thereto.
 19. An engine according to claim 18,wherein the holding means comprises a ceramic tube.
 20. An engineaccording to claim 14, wherein the cylinder forms part of a pipe forcarrying the exhaust gas.
 21. An engine according to claim 13, whereinthe trapping means comprises a pair of plates arranged in parallel and aplurality of wires arranged equidistantly in parallel to each other anddisposed midway between the plates.
 22. An engine according to claim 13,wherein the collecting electrode is earthed.
 23. An engine according toany claim 13, wherein the power means is capable of applying a potentialdifference of from −16 kV to +25 kV.
 24. An engine according to claim 1,wherein the trapping means is coated, at least in part, with thecatalyst.
 25. An engine according to claim 13, wherein the collectingelectrode is coated, at least in part, with a catalyst according toclaim
 1. 26. A method of combusting PM of <100 nm from a gasoline enginein CO₂ and/or H₂O from the exhaust gas at temperatures in excess of 500°C., which method comprising contacting the exhaust gas with a three-waycatalyst, trapping the PM and contacting the trapped PM with a catalystcomprising a supported alkali metal.
 27. A method according to claim 26,wherein the trapping step is by electrophoresis or filtration.
 28. Anengine according to claim 1, wherein the alkali metal is potassium. 29.An engine according to claim 7, wherein the support is a mixed oxide ofany two or more selected from the group consisting of alpha-alumina,zirconia and ceria.
 30. An engine according to claim 7, wherein thesupport comprises a mixed oxide and each component of the mixed oxide ispresent in an amount of from 10 wt % to 90 wt % by total catalystweight.
 31. An engine according to claim 30, wherein the mixed oxide isa binary mixed oxide and the ratio of the cationic components present isfrom 20:80 to 50:50.
 32. An engine according to claim 12, wherein themetal oxide is selected from the group consisting of alpha Al₂O₃,ZrO₂/MgO, ZrO₂/CaO/MgO and ZrO₂/Al₂O₃.
 33. An engine according to claim15, wherein the first cylinder comprises inner and outer surfaces and aninsulating layer is disposed therebetween, which first cylinder iscoaxially disposed within a second cylinder, the arrangement being suchthat the power means is capable of applying a potential differencebetween the coaxially disposed wire and the inner surface of the firstcylinder and between the outer surface of the first cylinder and thesecond cylinder.
 34. An engine according to claim 16, comprising aplurality of second cylinders arranged in parallel.
 35. An engineaccording to claim 1, wherein the elemental alkali metal is present inthe catalyst at from 5% to 15% by weight of the total catalyst.
 36. Anengine according to claim 1, wherein the elemental alkali metal ispresent in the catalyst at 10% by weight of the total catalyst.